Optical fiber is now used in a variety of telecommunication applications because of its small physical size and high bandwidth capacity. An optical fiber cable typically contains many optical fibers. The optical fibers can be contained in the cable in a variety of configurations, such as, for example, in an optical fiber ribbon, as a fiber strand or loosely enclosed in a buffer tube.
An optical fiber is a mechanically fragile structure. The optical signal transmission characteristics of an optical fiber can substantially degrade if the fiber is mechanically stressed. If a fiber is too severely mechanically stressed, the fiber can become non-functional for purposes of optical signal transmission in a telecommunication application.
It is not uncommon that an optical fiber cable containing an optical fiber or optical fibers will undergo handling or be exposed to a physical environment that can stress the fiber or fibers within the cable. For example, an optical fiber contained in an optical fiber cable can experience stress and strain when the cable is bent or stretched during winding on a reel for purposes of storage, or during or after installation along and over another surface, in a pipe or duct or suspended in air from vertical supports. Also, the fiber in a cable can be mechanically stressed if it is pinched between other cable components and because of the difference between the coefficients of thermal expansion for the optical fiber and the other components in the optical fiber cable containing the fiber.
When an optical fiber cable is bent, bending occurs along a neutral surface plane which is associated with the cable bending and extends along the longitudinal length of the cable. The intersection of the neutral surface plane with a cross-section of the cable is a neutral axis.
If an optical fiber cable is of uniform construction in all directions radially of its axis, the cable has the same rigidity, e.g., resistance to bending, in all directions transverse to the axis. However, if there are discrete components, such as strength members in portions of the cable, there are two preferred directions of bending in a preferred plane of bending or there may be more than one direction of bending in which the cable can be bent more easily than in other directions. Thus, there can be a longitudinal plane (MIN-BP) intersecting the cable in which minimum bending energy is required to bend the cable. As viewed in cross-section, a neutral axis called "NA.sub.Min " is associated with the bending of the cable in the MIN-BP, and NA.sub.Min is perpendicular to the MIN-BP and may intersect the cable axis. With such structure there is another such plane (MAX-BP) in which maximum bending energy is required to bend the cable, and there is a similar neutral axis called "NA.sub.Max " which is associated with the bending of the cable in the MAX-BP and is perpendicular to the MAX-BP but may not intersect the geometric center of the cable.
When a radially non-uniform optical fiber cable is subjected to bending forces, the cable will seek to orient and twist itself to cause bending to occur in the plane for which a minimum of energy is required to bend the cable, i.e., the MIN-BP. When a cable is bent in a particular plane, the material of the cable at opposite sides of the neutral surface plane associated with the plane of bending is respectively compressed and concave, and stretched and convex and in tension. During bending of the cable, any component in the cable which is free to shift radially of the cable, such as an optical fiber loosely received in the bore of the core, tends to migrate from the portion of the cable under tension or compression to the portion where strain is minimized. The cross-sectional area in the cable within which any loosely held fiber can move, the length of the fiber in relation to the cable and the plane in which the cable is bent determine the location in the cable where the fiber will become positioned as a result of the bending of the cable. If, during bending of a cable, a loosely held fiber within the cable becomes positioned away from the neutral surface associated with the bending of the cable, elongation or contraction stress can be applied to the optical fiber if other expedients are not employed. Although it is desirable that any loosely held fiber in the cable is positioned on or near the neutral surface associated with the expected bending of the cable, which in many circumstances will be in the MIN-BP, it is possible to reduce or eliminate such stress by suitably selecting the size of the bore and the excess of the fiber length with respect to the rectilinear length of the bore axis (EFL).
Prior art optical fiber cables have been designed to include features which control the behavior of the cable when subjected to bending and control the location of the neutral surface in bending to limit the stress on fibers in the cable. For example, the optical fiber cable of U.S. Pat. No. 4,844,575, incorporated by reference herein, includes two diametrically opposing strength members in the cable jacket to provide the cable with a MIN-BP having an associated neutral surface which intersects the center of the cable and the centers of the opposing strength members. Such cable can be bent most easily in either of two directions.
In addition, U.S. Pat. No. 4,836,639, incorporated by reference herein, discusses the problems of winding and unwinding a pipe or tube containing optical fibers around a drum and discloses an optical fiber cable which includes one or more strength members within the tube wall and optical fibers which assume positions at the inner wall of the jacket of the cable. The strength member(s) of the cable and the tube wall of the '639 patent position the neutral surface associated with bending of the cable in the MIN-BP near or coextensive with the position of the fibers in the cable with bending and so that when the tube is wound on a drum, the strength member or members are nearer the drum axis than the fibers, i.e., radially inwardly of the fiber. While the solution of the '639 patent can be useful when the tube is wound on a drum, the solution is not satisfactory when the tube is used in other applications, e.g., aerial applications, or when the optical fibers are within a core comprising elements, such as a buffer tube, strength members, armoring, etc. which is surrounded by the tube of the '639 patent as an outer jacket. Thus, in aerial applications the strength member is above the optical fibers, the loose optical fibers do not move significantly toward the strength member or the neutral axis described in the '639 patent.
Although the '639 patent indicates that only one reinforcing wire can be used, the '639 patent also indicates that the number of reinforcing wires should be greater than one in order to insure that the cable is wound around a drum in the intended direction. In fact, if only one reinforcing wire were used, the patent does not indicate how a preferred direction of bending would be obtained.
The inclusion of multiple strength members within a cable jacket is disadvantageous for several reasons. First, the arrangement of a plurality of strength members in the cable jacket makes the cable extremely stiff. An overly stiff cable makes handling and maneuverability of the cable difficult because substantial energy would be required to bend the cable in a plane other than the MIN-BP with a minimum of twisting, which often is desirable and required during and after installation of the cable. Also, the inclusion of multiple strength members in the jacket greatly increases the cable weight and the size of the cable in diameter and bulk to cause other undesirable inefficiencies. Further, the manufacturing step of extruding plastic over multiple strength members to obtain a desired jacket structure is complex and difficult. Finally, it is more difficult to secure aerial hardware to multiple strength members than to a single strength member in an aerial installation of a cable.
There are prior art cables suitable for aerial installation, see, for example, U.S. Pat. Nos. 4,097,119 and 5,095,176, incorporated by reference herein, which include metallic messenger wires which are connected to the main body of the cable by a thin web of jacket material and which can be used to suspend the cable securely from vertical supports. In this cable design, an additional longitudinal strength member, such as a reinforced rod or a metal sheath bonded to the jacket, is required in the core of the cable, because the messenger wires are not sufficiently coupled to the layers around the optical fiber to provide the cable itself with sufficient pulling and anti-compression resistance to minimize stress on the fibers in the aerial installation of the cable. In other words, the messenger wires do not provide a dual function of cable suspension and stress resistance. Also, the inclusion of strength members in the core or bonding of a metal sheath to the jacket can make the cable undesirably stiff.
Other optical fiber cables suitable for aerial installation, see, U.S. Pat. Nos. 5,125,063 and 5,448,670, incorporated by reference herein, include two diametrically opposed strength members embedded in a jacket which encloses a central tube loosely surrounding optical fibers. In an aerial installation, these cables are either clamped directly to a vertical support, or to a separate and independent messenger wire which extends along a series of vertical supports and which connects to and carries the weight of the installed cable. Such a cable design is inefficient because two strength members are required and because of the disadvantages described hereinbefore.
Similarly, the optical cable suitable for aerial installation described in U.S. Pat. No. 4,798,443, incorporated by reference herein, which includes a plurality of non-metallic reinforcing members embedded in the jacket and extending generally parallel to the axis of the cable, and which cable can be clamped directly to the vertical supports in an aerial installation, has some of the same disadvantages associated with the cables of the '670 and '063 patents. Although the '443 cable design provides for a plurality of optical elements to minimize strain on the fibers in installation, where each optical element comprises several buffer tubes loosely carrying individual fibers and disposed around a non-metallic central member, this design may be more difficult and expensive to manufacture and access to the fibers at midspan of the cable is also more difficult.
Therefore, there exists a need for an optical fiber cable which is compact, has a small diameter, is lightweight, efficiently protects fibers loosely contained therein from mechanical stress in an aerial installation of the cable, which not only provides a preferred bending plane, i.e., the MIN-BP, but also allows for relative ease of bending of the cable in a plane other than the MIN-BP as compared to prior art cables and provides preferred directions of bending with respect to the MAX-BP.